Communities and ecosystems | AQA GCSE Biology Notes

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The key idea

Organisms in a community depend on each other and on abiotic conditions.A change to one factor can affect an entire food web.

A simple food webArrows show energy transfer from food to consumer.

Revision notes

The bit that matters

Learn the process in clean chunks. If a sentence explains a cause, make sure you can say the effect too.

Levels of organisation

An ecosystem is the interaction of a community of living organisms with the non living, or abiotic, parts of their environment.A population is all the organisms of one species in a habitat, and a community is all the populations of different species living together.Within a community organisms depend on each other for food, shelter, pollination and seed dispersal, which is called interdependence.

Competition

Organisms compete with each other for the resources they need to survive and reproduce.Plants compete for light, water, space and mineral ions from the soil.Animals compete for food, water, mates and territory.To survive in a community an organism must obtain the resources it needs in the face of competition from other species and members of its own species.

Abiotic and biotic factors

Abiotic factors are non living factors such as light intensity, temperature, moisture, soil pH, wind intensity and carbon dioxide or oxygen levels, which can affect the distribution of organisms.Biotic factors are living factors such as the availability of food, new predators or pathogens arriving, and competition between species.A change in one factor can affect the whole community.

Adaptations

Adaptations are features that enable an organism to survive in the conditions where it lives.Structural adaptations are physical features such as thick fur or a large surface area.Behavioural adaptations are actions such as migration.Functional adaptations relate to processes such as conserving water.Extremophiles are organisms adapted to live in very extreme conditions such as high temperature, pressure or salt concentration.

Key terms

Definitions to learn

Ecosystem

The interaction of a community of organisms with the abiotic parts of their environment.

Community

All the populations of different species living together in a habitat.

Population

All the organisms of one species living in a habitat.

Interdependence

When species in a community depend on each other to survive.

Abiotic factor

A non living factor such as temperature or light intensity.

Biotic factor

A living factor such as food availability, predators or competition.

Worked example

A disease reduces the number of rabbits in a food web. Suggest one effect on foxes and one effect on grass.

Foxes have less prey available.

Grass is eaten by fewer rabbits.

Predict changes from these links.

Final answer

Fox numbers may fall and grass abundance may rise.

Exam habit

Follow the food web chain in every direction: changes ripple upward and downward.Abiotic factors are non-living; biotic are living — give a specific example of each when asked.

Watch out

Food webs show interconnected relationships, so explain the chain of effects.

Examiner tips

How to score full marks

1Be precise with the difference between a population, a community and an ecosystem.
2When listing what organisms compete for, give the correct list for plants or for animals, not a mixture.
3Classify a named factor correctly as abiotic, non living, or biotic, living, before explaining its effect.

Practice questions

Try these yourself

Open each answer only after you have explained the full biological process.

1Give two abiotic factors affecting a community.

Mark scheme

1.Choose non-living environmental factors.

Any two from: light intensity, temperature, moisture, soil pH, wind or carbon dioxide level.

2What is interdependence?

Mark scheme

1.Describe reliance between species.

When organisms rely on each other for resources such as food, shelter or pollination.

3Why can introducing a new predator affect several populations?

Mark scheme

1.Follow multiple feeding relationships.

It changes prey numbers and therefore affects their food sources and other predators.

4Define the term community.[1 mark]

Mark scheme

1.State it is all populations in a habitat.

All the populations of different species living together in a habitat (1)

5State two resources that plants compete for.[2 marks]

Mark scheme

1.Recall the plant resource list.

Any two from: light (1); water (1); space (1); mineral ions from the soil (1)

6Give one abiotic and one biotic factor that could affect the distribution of organisms.[2 marks]

Mark scheme

1.Choose one non living and one living factor.

Abiotic: any one from light intensity, temperature, moisture, soil pH (1); biotic: any one from food availability, new predators, new pathogens, competition (1)

7Explain what is meant by interdependence in a community.[3 marks]

Mark scheme

1.Give the idea and an example.

Interdependence is when species in a community depend on each other to survive (1); for example for food or shelter (1); so that if one species is removed it can affect others in the community (1)

8A new predator is introduced to a stable ecosystem and the population of a small mammal it hunts falls sharply. Explain how this single change could affect the whole community.[4 marks]

Mark scheme

1.Trace the knock on effects through the food web.
2.Refer to interdependence.

The new predator eats the small mammals so their population falls (1); animals that competed with or ate the same food as the mammal may increase or decrease (1); plants that the mammals fed on may increase (1); because the species are interdependent a change to one population affects many others in the community (1)

9Describe three structural adaptations of a polar bear that help it survive in Arctic conditions.[3 marks]

Mark scheme

1.Give three specific features and link each to survival.

Any three from: thick layer of fat/blubber (1) provides insulation against cold (1); thick white fur (1) provides camouflage on snow for hunting (1); large paws (1) spread weight on ice and paddle in water (1); small surface area to volume ratio (1) reduces heat loss (1).

10Explain why two species competing for exactly the same resources in the same habitat cannot both survive long-term.[4 marks]

Mark scheme

1.Apply competitive exclusion.
2.Predict the outcome.

Both species compete for the same food, space or other resources (1); the better-adapted species will obtain more of the resource (1); the less competitive species will decline in numbers (1); eventually it will be excluded from the habitat or go locally extinct (1); this is called competitive exclusion.

11Explain what is meant by an extremophile and give one example of an adaptation that allows organisms to survive in extreme environments.[2 marks]

Mark scheme

1.Define extremophile.
2.Give a named example with an adaptation.

An extremophile is an organism that is adapted to survive in extreme conditions such as very high temperature, high salt concentration or high pressure (1); for example thermophilic bacteria in hot springs have enzymes with a high optimum temperature that do not denature in the heat (1).

12A graph shows the populations of lynx and hares over several years, with both populations showing regular cycles. The hare population peaks before the lynx population. Explain this pattern in terms of predator-prey interactions.[5 marks]

Mark scheme

1.Explain how hare numbers affect lynx numbers.
2.Explain how lynx numbers then affect hare numbers.

When hare numbers rise there is more food for lynx so lynx numbers increase (1); the increased lynx population predates more hares, causing hare numbers to fall (1); as hares become scarce lynx have less food and lynx numbers fall (1); with fewer predators hare numbers can rise again (1); the cycle then repeats, with the lynx peak lagging behind the hare peak because it takes time for the predator population to respond (1).

Official exam-board sources

Specification Past papers

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